[pron]

The reason most "serious" or important software is written for the JVM these days is because it gives you an unparalleled combination of performance, productivity, and observability. There's almost no competition if these things are what you need. The problem isn't so much why pick Java among the alternatives, but that there are hardly any alternatives.

[AlotOfReading]

You have to put on some very narrow lenses to argue that "most" serious software is written for the JVM. Operating systems, compilers, browsers, databases, planes, cars, the transaction processing for at least one major payment processor, major cloud services, etc predominantly use other languages.

I don't think there's any single language that most serious software is written in.

[pron]

C, C++, and C# are obviously also major players in "serious software", but you can estimate the volume of software through the number of people involved (e.g. https://www.devjobsscanner.com/blog/top-8-most-demanded-prog...). If Java doesn't have an outright majority, it has an obvious plurality. And again, there aren't many alternatives.